Using BPL communications on MV power lines requires repeating or regenerating signals at various intervals to maintain sufficient signal strength to reach the signal destination. Such signal regeneration may be done by BPL modems connected to the MV power line. When multiple devices try to communicate over a single physical line, the devices must follow a specified scheme to share the physical resource and to avoid interfering with each other. There are two such schemes commonly used, one referred to as Time Division Duplexing (“TDD”), and the other as Frequency Division Duplexing (“FDD”).
TDD is a scheme whereby devices split up a period of time T (seconds) into N divisions, with each device being given T/N (seconds) of the total time T in which to communicate over the single line. Each device waits for its specific time slot and when its turn arrives, the device uses the full frequency band available to communicate.
FDD is a scheme whereby devices split up the total frequency band F (measured in Hertz), into N divisions, with each device being given F/N (Hertz) of the total F band in which to communicate. Each device communicates as required (as opposed to TDD where a device only communicates in its own designated time division) but only in its allocated frequency band (as opposed to TDD where a device uses the full frequency band to communicate).
When using FDD (which is the most efficient way of building large networks) BPL modems require at least two internal modems and corresponding ports, one for upstream communication along the MV power line, and one for downstream communication along the MV power line. Each port is connected to a MV coupler used to couple the signal from the internal modem to the MV power line.
FIG. 1 shows a typical BPL access system over MV power line 100 utilizing FDD for repeating the BPL signal. BPL modem 110 has two ports, 120 and 130, for input and output, for upstream and downstream communications, respectively. Each port 120, 130 is connected to a coupler 140, 150.
BPL modem 110 also acts as a repeater. BPL signal 1 is received and “repeated” (i.e. the signal is regenerated and retransmitted) and sent out as BPL signal 2 and vice versa. BPL Modem 10 typically includes two internal modems (and may have more) for FDD repeating. In FDD communications, when the signal is repeated a different frequency band is used for the upstream and downstream directions of communication, respectively, but BPL modem 110 is transmitting/receiving constantly (unlike TDD communications wherein a time slot is assigned).